This invention is related to an active hand control system of the type wherein manual control input devices such as control sticks employed in a servo-coupled control system are provided with an electrically generated and controlled simulated variable rate feel. In particular, the invention relates to a system having a control stick such as is employed in aircraft, which is servo-coupled to the control system of the aircraft through electronics and a motor mechanism.
The electronics and motor mechanism of these types of systems provide a simulated feel to the control stick when in operation which is similar to that of a purely mechanical system. Yet more specifically, in these types of systems when the control stick thereof is positioned near a null or center position, it begins to oscillate due to the high gain in the torque versus position curve used to achieve breakout forces by the motor which is driving and is in turn driven by the control stick, as well as oscillations caused by gear backlash at connection of the control stick to the motor. The system in accordance with the invention eliminates such oscillations at about the null position for the control stick without degrading the force and feel characteristics of the control stick.
Servo-control technology is well developed as applied in the field of robotics. In particular, electrical motor and servo-control systems have been developed and employed in the past in the design of robotic hand controllers which are capable of reflecting forces experienced at the robotic end, back to, for example, a human operator.
One example of the type of control system to which the present invention is directed is disclosed in U.S. Pat. No. 4,150,803 which teaches a control stick for an operator having an electro-simulated variable rate feel. The simulated feel of the device of U.S. Pat. No. 4,150,803 is provided by a system which generates for the operator at the control stick the proper force and feel characteristics when used to command a boom employed during refueling operations between aircraft in flight.
These types of controllers generally involve the use of a control stick which is actuatable in at least two mutually perpendicular planes to provide both vertical and lateral control of the device being controlled. An example of such a control stick is shown in U.S. Pat. No. 3,270,260, in which the control stick of the device has electrical pickup means in plate form, for example, capacitance bridge circuits arranged in a symmetrical arrangement such that a force exerted on the stick having a symmetrical arrangement of circuits unbalances them and produces a signal which is proportional to the force being exerted on the stick. The signals generated by the electrical pickups are used to generate output signals through appropriate electronics, with the electronics being used to command a control actuator to effect control of the device being controlled.
Typically, in these systems the control stick is mechanically coupled at its axis to at least one motor which applies either a resisting force on the control stick or, response to a signal produced by sensors that detect forces applied to the device surfaces being controlled, drives the control stick and in turn, drives the device being controlled to alleviate forces generated by the device under control as felt by the operator.
These types of controllers are particularly desirable for use in the operation of modern day aircraft, in particular, in the form of control sticks or yokes. In operation, the devices as used in the cockpit of aircraft are typically designed to exhibit some desired force versus displacement characteristics to the user whereby the magnitude of the control stick displacement is proportional to the force applied. The controller produces as its output an electrical signal corresponding to the control stick position, and the signal is used to control the aircraft through the action of various motors and mechanical means, in a manner which is well known to those of ordinary skill in the art and is conventional. Thus, in use such systems provide an electronically controlled manual input control stick having force and feel characteristics like those of purely mechanically linked systems. These applications in aircraft are typically referred to as "fly by wire" applications. Examples of presently existing applications of this technology are the systems employed in the Airbus A300 Transport Aircraft, the General Dynamics F16 Fighter aircraft and the NASA Space Shuttle.
In the past, in order to enhance or improve the force and feel characteristics of such simulated feel control sticks, controller electronics were employed in combination with sensors for detecting the position of the control stick. A signal was generated and processed by controller electronics to drive a motor which in turn drove the control stick to simulate the feel of a mechanical system. Such controller electronics also included input from the system being controlled such as, for example, an auto pilot system reflecting any external forces acting on the flight control surfaces of the aircraft which would, as a result of feedback and input from the surfaces, be input into the controller electronics to be reflected through the motor connected to the control stick at the user.
The use of a feedback loop in which the position of the control stick is detected and a signal resulting therefrom is processed by controller electronics to result in control of a motor to which the control stick is connected to achieve force and feel characteristics typical of a mechanical system is known. In accordance with one improvement as disclosed in copending application no. (attorney docket no. A34-14307(15347-153)), which is incorporated by reference herein and which was concurrently filed herewith, it is recognized that the detection of the position of the control stick fails to fully achieve the desired force and feel characteristics typical of mechanical systems. It is taught in said copending application that a mechanical system can be more fully simulated by also detecting the amount of force being exerted on the control stick, and processing both the position as well as the force signal by controller electronics. This is done to generate a control signal for the motor to which the control stick is connected which reflects both force and position as part of the feedback loop. By reflecting force the loop, the effects of external operational non-linearities resulting from the electro-mechanical connections of the manual control stick are eliminated.
Notwithstanding the improvement disclosed in said copending application, when the control stick of such a system is located about its null position, a fairly large motor torque causes the hand controller to oscillate at null unless the position signal is exactly at zero when fed to the feedback loop which processes such a signal through the controller electronics. Traditionally, the oscillations at null have been corrected by providing a conventional rate dampening signal manipulation which adds stability to the control loop. However, the impact of providing significant rate damping is a degraded transient response. More particularly, excessive rate dampening, in accordance with the prior art in an active hand controller, makes the hand controller feel viscous. The invention addresses the problems of providing such rate dampening with a degraded transient response.